1. Field of the Invention
The present invention relates to fuel cell systems operating at or near ambient pressure. In particular, the present invention relates to fuel cell systems operating at or near ambient pressure with partial air humidification, and methods of operating such systems.
2. Description of the Related Art
Fuel cells are known in the art. Fuel cells electrochemically react a fuel stream comprising hydrogen and an oxidant stream comprising oxygen to generate an electric current. Fuel cell electric power plants have been employed in transportation, portable and stationary applications.
Water management issues are critical in polymer electrolyte membrane (PEM) fuel cell operation. Humidification of the membrane is required to maintain optimal performance. As the water content of the membrane falls it loses the ability to transport protons, its electrical resistance increases, and fuel cell performance decreases and membrane failure may occur. To ensure adequate humidification of the membrane the reactant streams supplied to the fuel cell stack are typically humidified. At the same time, water present at the cathode—both product water produced at the cathode and water transported across the membrane from the anode as a result of electroosmotic drag—must be removed, otherwise flooding of the cathode flow field can occur.
Various approaches have been used to humidify the reactant streams supplied to fuel cells. For example, reactants can be humidified by flowing the reactant stream and liquid water on opposite sides of a water permeable membrane. Product water from the fuel cell can be condensed from the reactant exhaust streams and then used for humidification. Water vapor or atomized water droplets may be injected into the reactant streams, as well. The oxidant stream may also be heated and humidified by flowing it and the oxidant exhaust stream on opposite sides of a water permeable membrane in a combined heat and humidity exchange apparatus. Examples of the latter apparatus are described in U.S. Pat. No. 6,007,931 and commonly assigned U.S. Pat. No. 6,416,895 B1.
Of course, water management issues need to be considered in the context of other system requirements. In many applications, such as automotive systems, for instance, high power density (power output capability per unit volume) operation is desirable, as is the ability to operate partial loads. Under such dynamic load conditions, maintaining a proper water balance in the fuel cells can be particularly challenging.
One approach to achieve these ends is to operate the fuel cell stack at higher pressure. The higher partial pressure of reactants in the fuel cells supports higher power density operation and higher operating temperatures. Higher pressure drops across the reactant flow fields also enables mechanical removal of liquid water from the fuel cells. However, this approach requires expensive air pressurization and hydrogen recycle equipment that increase the cost, complexity and size of the power plant and also represent a significant parasitic power loss. Humidification systems for the reactants similarly increase the cost, complexity and size of the power plant. The cost of stack components capable of operating at high pressures, such as seals, for example, may also be increased. Furthermore, liquid water management issues can cause unstable cell operation under dynamic load conditions.
An alternative approach employs wicking or similar passive means to provide water to the membrane. PEM fuel cell power plants developed by UTC Fuel Cells, LLC, for example, employ this approach. Porous water transport plates adjacent porous anode and/or cathode support layers facilitate water transport to the anode and/or cathode surfaces. These power plants can be operated at near-ambient pressures, which can reduce the cost and power loss associated with an air compressor.
This approach has some disadvantages as well. For example, the water transport plates and coolant loops tend to be complex. Operation and control systems are also complicated: product water enters the coolant loop and must be removed; pressure differentials are created between the reactant flow fields and coolant water circulation passages to assist water transport through the porous support layers and cells; and/or, dual coolant loops may be employed to ensure water balance at higher ambient temperatures. These power plants also require complex systems for cold start capability, such as, for introducing methanol or ethanol into coolant passages on shutdown to prevent water trapped therein from freezing, or introducing a non-volatile organic antifreeze solution into the water transport plates. Further, operation using zero relative humidity reactant streams with a PEM fuel cell having a porous water transport plate eventually causes a drying out of the PEM electrolyte; US 2002/0068214 A1 discloses that the loss of water from the electrolyte may be restricted by incorporating anode and/or cathode electrolyte dry-out barriers. Such efforts at maintaining efficient water balance involve additional cost, weight, volume burdens, fuel cell performance penalties, and often require complex control apparatus.
U.S. Pat. No. 6,451,470 B1 and CA 2,342,825 A1 disclose gas diffusion layers having a gas permeability gradient perpendicular to the membrane that inhibits the diffusion of water from the membrane (a “gas diffusion barrier”, or GDB). This permits fuel cell operation without external humidification of the reactants. The GDB structure also allows air-cooling of the fuel cell by supplying air to the cathode flow fields at relatively high stoichiometries. The '825 application, for example, discloses adopting air ratios, i.e., stoichiometries, of 8-70.
However, ambient pressure fuel cell systems using such passive water management systems do not appear suitable for high power density applications and/or operating at higher temperatures. For example, the '470 patent discloses that the maximum operating temperature of fuel cells employing a GDB is about 75° C. if air is supplied at ambient pressure. According to the '470 patent, at higher temperatures, a GDB having a sufficiently low effective diffusion coefficient for water would no longer ensure sufficient diffusion for reactant gases, in particular oxygen; although increasing air pressure permits increased operating temperatures (col. 3, lines 29-42). Indeed, the system disclosed in the '470 patent demonstrated a drastic drop in performance above a power density of 503 mA/cm2 at near-ambient air pressure (col. 7, lines 50-60). Published results of stacks employing UTC Fuel Cell porous bipolar plates disclose optimized performance at 65° C. (see, D. J. Wheeler et al., J. New Mat. Electrochem. Systems 4, 233-238 (2001)).
It is desirable to have a fuel cell electric power plant that can maintain adequate water balance in the fuel cells at higher current densities and temperatures, without requiring liquid water removal or a pressurized air supply. The present invention addresses the disadvantages of conventional fuel cell power plants and provides further related advantages.